1. Field of the Invention
The present invention relates to an image recording apparatus designed to record images by using recording heads, each having a plurality of recording elements arranged in an array, a method of calculating shifts of record positions of images, and a method of recording measured patterns.
2. Description of the Related Art
In general, image recording apparatuses, such as ink jet printers, have a recording head that has a plurality of recording elements arranged, forming an array, and discharges ink to a recording medium being carried, thereby recording an image on the recording medium. Any color printer has, for example, four nozzle arrays arranged in the direction of carrying a recording medium and configured to discharge inks of four different colors, respectively, thereby to record a color image.
In such an image recording apparatus, ink drops discharged from each recording element, array will land the recording mediums at wrong positions if the recording element array is held at a wrong position or if the ink drops are discharged at a wrong time. Consequently, the color images of different colors will shift from one another, blurring the edges of the color image or lowering the quality of the color image due to the spreading of color inks.
In order to eliminate such shifts of image positions, thereby to record a high-quality color image, the direction and distance in and by which the color images of different colors shift in position must be measured. A method of measuring the shifts of image positions is proposed in, for example, Patent Document 1. In this method, a prescribed measuring pattern is recorded (or printed) and photographed, generating image data. This image data is processed by a computer, thereby calculating the direction and distance in and by which the color images of different colors shift in position.
In this method, a black-ink recording head unit 16k and a cyan-ink recording head 16c are driven, at every two other nozzles, as is shown in FIG. 17A. As a result, first measuring patterns p1 and p2 are recorded on a recording medium, as shown in FIG. 17B. The measuring patterns p1 and p2, each being an array of dots, extend parallel to each other, in the direction orthogonal to the direction in which the recording medium is carried. The first pattern p1 is constituted by black ink dots, while the second pattern p2 by cyan ink dots. All ink dots shown in FIG. 17B assume correct positions on the recording medium, because they have been formed by an image recording apparatus perfectly adjusted. In practice, however, ink dots assumes positions shifted from those positions shown in FIG. 178.
A second example of the method will be explained with reference to FIGS. 18A and 18B. FIG. 18A shows the black-ink recording head unit 16k and cyan-ink recording head 16c. FIG. 18B shows two patterns p11 and p12, each being an array of dots, extend parallel to each other, in the direction in which the recording medium is carried. The pattern p11 is recorded by the head unit 16k, while the pattern p12 by the head unit 16c. On the recording medium, a pattern p13 is formed, which is constituted by the patterns p11 and p12. The measuring pattern shown in FIG. 18B indicates the positions the dots assume when the color images of different colors do not shift at all. Each dot is spaced apart from any other so that its coordinates may be determined well. In this case, the recording element arrays shown in FIG. 18A are driven, at every two other nozzles, recording a color image.
Next, in order to determine the coordinates of each dot from these patterns, the patterns p1 and p2 or the patterns p11 and p12, photographed by a read unit, are input to a computer. The computer generates image data that can be processed.
The read unit that reads the patterns is, for example, such a flat bed scanner 101 shown in FIG. 19. The flat bed scanner 101 has a housing 102 and a line sensor 104. The housing 102 has a reading surface 103. On the reading surface 103, the recording medium having the patterns p1 and p2 or patterns p11 and p12 is set. The line sensor 104 scans the recording medium, in scanning direction n, reading the patterns p1 and p2 or patterns p11 and p12 or acquiring them as image data. The image data is processed, using the image processing technique known in the art, determining the coordinates of each of the dots constituting the patterns p1 and p2 or patterns p11 and p12.
To calculate the direction and distance in and by which the two patterns p1 and p2 or patterns p11 and p12 shift, the coordinates of the gravity center of each of the patterns p1 and p2 or patterns p11 and p12 are calculated as points representing the positions of the patterns. Note that the gravity center is a point that represents the position of the pattern. In this case, the coordinates of the gravity center is the average of the coordinates values of the dots constituting each pattern (p1, p2, p11 or p12), or so-called “average coordinates.”
FIG. 17B illustrates the case where the patterns p1 and p2 have gravity center G1 and gravity center G2, respectively. The shift between the position of the gravity center G1 of the pattern p1 and the position of the gravity center G2 of the pattern p2 is determined. The patterns p1 and p2 are recorded at correct positions, or do not have a recording position shift (zero shift) if their gravity centers are identical on the x axis and spaced apart by a prescribed distance on y axis as is shown in FIG. 17B.
If the pattern p2 shifts to the right from the pattern p1 in the state shown in FIG. 17B, the shift of the gravity center G2 on the x axis with respect to the gravity center G1 assumes a positive value. If the pattern p2 shifts upwards from the pattern p1 in the state shown in FIG. 17B, the shift of the gravity center G2 on the y axis with respect to the gravity center 51 assumes a positive value. The direction in which the recording element arrays extend is not always orthogonal to the direction in which the recording medium is carried. In view of this, the direction in which the recording medium is carried during the image recording is defined as y-axis direction, and the direction of carrying the recording medium is x-axis direction.
In FIG. 18B, the patterns p11 and p12 similarly have gravity center oil and gravity center G12, respectively. In FIG. 18B, the y axis defines the direction in which the recording medium is carried during the image recording, and the x axis orthogonal to the y axis defines the direction in which the recording element arrays are adjusted in position. The shift between the gravitation center G12 of the pattern p12 and the gravitation center G11 of the pattern p11 is then determined.
If the gravity centers of the measuring patterns p11 and p12 are spaced apart by a prescribed distance L in the x-axis direction as shown in FIG. 18B, no recording shifts exist, or the recording error is zero. In this state, if the pattern p12 shifts to the right from the pattern p11, the shift of the gravity center G12 on the x axis with respect to the gravity center G11 assumes a positive value. If the pattern p12 shifts upwards from the pattern p11 in the state shown in FIG. 18B, the shift of the gravity center G11 on the y axis with respect to the gravity center G12 assumes a positive value.
In the image recording apparatus, the positions of the recording element arrays and the timing of recording an image is adjusted so that the relative shift between the recording positions of the measuring patterns may be reduced to zero in terms of the relative position shift.
In order to determine the distances by which the measuring patterns shift in the x-axis direction or y-axis direction at high accuracy, it is important to mount the recording medium on the reading surface in a correct orientation so that the direction in which the recording medium is carried during the image recording may be aligned with the direction in which the line sensor scans the medium during the image reading.
In order to determine, at high accuracy, the distance L between the gravity centers G11 and G12, as measured in the x-axis direction, in such a state as shown in FIG. 18B, the scale the imaging apparatus has must be accurate. If the scale has an error of ±E %, the distance determined will be erroneously determined to be L×(100±E)/100. This error increases as the distance L between the gravity centers G11 and G12 increases.